A065896 Number of composites <= 2*n.

0, 1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 14, 16, 18, 19, 20, 22, 24, 25, 27, 28, 29, 31, 32, 34, 36, 37, 39, 41, 42, 43, 45, 47, 48, 50, 51, 52, 54, 56, 57, 59, 60, 62, 64, 65, 67, 69, 71, 72, 74, 75, 76, 78, 79, 80, 82, 83, 85, 87, 89, 91, 93, 95, 96, 98, 99, 101, 103, 104, 105

Offset: 1

Author

Labos Elemer, Nov 28 2001

Keywords

• nonn

Examples

			a(9) = 10, since 18 is the 10th composite number: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18.
		

Links

• Harry J. Smith, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A002808, A000720, A005843, A065855.

Programs

Haskell

a065896 = a065855 . (* 2)  -- Reinhard Zumkeller, Oct 14 2014

Mathematica

Table[2*n-(PrimePi[2*n])-1, {n, 1, 128}]

PARI

{ for (n=1, 1000, f=2*n; a=f - primepi(f) - 1; write("b065896.txt", n, " ", a) ) } \\ Harry J. Smith, Nov 04 2009

Formula

a(n) = A065855(2*n). - Reinhard Zumkeller, Oct 14 2014

Extensions

More precise definition from Michel Marcus, Mar 07 2021~